Iridium-catalyzed regio- and enantioselective N-allylation of indoles

Article abstract of DOI:10.1002/anie.200904338

Iridium is blind to C: Highly regio- and enantioselective, iridium-catalyzed N-allylations of indoles, which complement the more common reactivity of indoles as carbon nucleophiles, have been developed (see scheme). These reactions form highly enantioenri

Full text of DOI:10.1002/anie.200904338

Angewandte
Chemie
DOI: 10.1002/anie.200904338
Synthetic Methods
Iridium-Catalyzed Regio- and Enantioselective N-Allylation of
Indoles**
Levi M. Stanley and John F. Hartwig*
Chiral indole architectures are present in a wide variety of
natural products and have been identified as promising lead
compounds in medicinal chemistry.[1] Therefore, extensive
effort has been dedicated to synthesizing enantioenriched
indole derivatives by catalytic, enantioselective reactions,[2]
such as 1,2-additions to carbonyl compounds and imines,[3]
additions to electron-deficient alkenes,[4] and allylation reac-
tions.[5] However, the indole acts as a carbon nucleophile in
each of these reactions; catalytic, enantioselective reactions at
the nitrogen atom of an indole are rare, but would provide
access to an array of enantioenriched, heterocyclic architec-
tures.
One approach to overcome the greater nucleophilicity of
the C3-position of indole, relative to that of N1, is to install an
electron-withdrawing substituent at C2. Such a substituent
tempers the nucleophilicity at C3, increases the acidity of the
readily converted into monoamine reuptake inhibitors,[9] the
indole core of integrin avb3 inhibitors,[10] and highly substi-
tuted dihydropyrrolo[1,2-a]indoles.
Initial studies to develop catalytic, enantioselective N-
allylations of indoles were guided by work from our
laboratory on the N-allylation of more acidic and nucleophilic
benzimidazoles, imidazoles, and purines catalyzed by the
metallacyclic iridium–phosphoramidite complexes [Ir(cod)-
(k2-L1)(ethylene)] (1)[11] and [Ir(cod)(k2-L2)(ethylene)]
(2).[12] To test whether iridium-catalyzed N-allylation of
ꢀ
N H bond, and can be used either for additional trans-
formations or removed after reaction at the nitrogen center.
Even by following this approach, catalytic, enantioselective
reactions of indoles at the nitrogen center are rare. Recent
reports by Bandani et al.[6] and Chen et al.[7] have shown that
asymmetric alkylation of the indole nitrogen atom occurs with
modified cinchona alkaloids, but just one report of enantio-
selective reactions catalyzed by a transition-metal complex
that occur at an indole nitrogen atom has been published. In
this work, N-substituted indolopyrrolocarbazole derivatives
were synthesized by palladium-catalyzed asymmetric allylic
alkylation of bis(indole) lactam pro-aglycons.[8]
The potential of enantioselective N-allylation of indoles
to create an entry into biologically active indole derivatives,
particularly with catalysts that preferentially form chiral,
branched products from linear allylic esters, led us to
investigate iridium-catalyzed, asymmetric, N-allylation of
indoles. We report herein the synthesis of enantioenriched,
branched N-allylindoles from the reactions of 2-subsituted, 3-
substituted, and 2,3-disubstituted indoles with achiral linear
allylic carbonates in the presence of a single-component
iridium catalyst [Eq. (1)]. The enantioenriched products are
indoles could occur, we studied the reaction of ethyl indole-
2-carboxylate (4a) with allylic carbonates. After conducting
reactions under various conditions with different catalysts and
different carbonate derivatives (see Table S1 in the Support-
ing Information for details), we found that the allylation of 4a
with tert-butylcinnamyl carbonate in the presence of 2 mol%
of metallacycle 2 and 10 mol% Cs2CO3 occurred exclusively
at N1 with excellent branched-to-linear selectivity (5a/6a =
97:3). This reaction was conducted with the single-component
iridium catalyst 2 to eliminate potential complications that
would arise from the generation of the metallacyclic catalyst
by base[13] in a system containing a weakly basic nucleophile
and carbonate base. The branched N-allylindole (R)-5a was
isolated in 89% yield and 99% ee [Eq. (2) and Table 1,
entry 1]. The absolute configuration of the N-allylindole 5a
[*] Dr. L. M. Stanley, Prof. Dr. J. F. Hartwig
Department of Chemistry
University of Illinois Urbana-Champaign
Urbana, IL 61801 (USA)
Fax: (+1)217-244-8024
E-mail: jhartwig@illinois.edu
Homepage: http://www.scs.uiuc.edu/hartwig/
[**] We thank the NIH for financial support of this work (NIH GM55382
to J.F.H. and GM84584 to L.M.S.) and Johnson-Matthey for gifts of
iridium salts.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904338.
Angew. Chem. Int. Ed. 2009, 48, 7841 –7844
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Communications
Table 1: Iridium-catalyzed enantioselective allylation of ethyl indole-2-
carboxylate 4a with allylic carbonates 3a–k.[a]
more forcing reaction conditions. Although the branched-to-
linear ratio was only 87:13, the allylation product 5k was
isolated as a single regioisomer in 54% yield and with 99% ee
when the reaction was conducted at 508C in the presence of
4 mol% 2 (Table 1, entry 11).
The range of indoles that undergo the enantioselective N-
allylation in the presence of catalyst 2 is summarized in
Table 2. Various ethyl indole-2-carboxylates containing elec-
Entry
3 (R)
5
5/6[b]
Yield [%][c]
ee [%][d]
Table 2: Iridium-catalyzed N-allylation of 2-substituted, 3-substituted,
and 2,3-disubstituted indoles 4b–p with tert-butyl cinnamyl carbonate
3a.[a]
1
2
3
3a (Ph)
5a
5b
5c
5d
5e
5 f
5g
5h
5i
97:3
99:1
97:3
91:9
98:2
95:5
91:9
99:1
94:6
77:23
87:13
89
88
87
85
95
72
85
86
92
70
54
99
99
99
99
98
96
99
99
99
98
99
3b (4-MeOC6H4)
3c (4-BrC6H4)
3d (4-F3CC6H4)
3e (3-MeOC6H4)
3 f (2-MeOC6H4)
3g (2-furyl)
4[e]
5
6[f]
7[f]
8[f,g]
9[f]
10[f]
11[e]
=
3h ((E)-CH CHCH3)
3i (n-propyl)
3j (CH2OBn)
3k (cyclohexyl)
5j
5k
Entry
4
R1
R2
R3
OMe 5l
5
5/6[b] Yield [%][c] ee [%][d]
[a] See the Supporting Information for experimental details. [b] Deter-
mined by 1H NMR analysis of the crude reaction mixture. [c] Yield of
isolated 5. [d] Determined by chiral HPLC methods. [e] Used 4 mol% 2
as the catalyst. [f] Reaction was run at room temperature. [g] Enantio-
meric excess was determined after reduction with LiAlH4.
1
2
3
4
5
6
7
8
4b CO2Et
4c CO2Et
4d CO2Et
4e CO2Et
4 f CHO
4g
4h
4i
4j
4k
4l
4m Ph
4n Ph
4o Ph
4p
H
H
H
H
97:3 88
99
99
99
99
99
97
96
96
96
97
99
97
99
99
98
F
Cl
5m 94:6 84
5n 95:5 91
NO2 5o 94:6 90
H
H
H
H
H
H
H
H
H
H
H
H
5p 98:2 89
5q 96:4 84
5r 94:6 87
5 s 98:2 88
5t 96:4 93
5u 99:1 21
5v 98:2 82
5w 96:4 93
5x 98:2 95
5y 98:2 89
5z 98:2 88
H
H
H
H
H
CO2Me
CHO
C(O)Me
CN
Ph
parallels that of the products from the addition of other
nucleophiles that appear to react by backside attack onto the
allyl ligand of the intermediate allyliridium complex.[14]
The results of reactions of ethyl indole-2-carboxylate with
a series of achiral, linear allylic carbonates under these
reaction conditions are shown in Table 1. The reactions of 4a
with electron-rich, electron-neutral, and electron-deficient
substituted cinnamyl carbonates gave the corresponding N-
allylindole products 5b–f in high yields with a range of 91:9 to
99:1 branched-to-linear selectivity and 96–99% enantiomeric
excess (Table 1, entries 2–6). The reaction of 4-(trifluoro-
methyl)cinnamyl tert-butyl carbonate 3d with 4a required a
higher loading of catalyst 2 (4 mol%), but gave product 5d in
85% yield with a 91:9 branched-to-linear ratio and 99% ee
(Table 1, entry 4). Furthermore, this reaction occurred with-
out isomerization of the chiral 1-arylallyl product into the
corresponding achiral 1-arylprop-1-enyl product; an analo-
gous isomerization was observed during the allylation of
benzimidazoles and purines with electron-deficient cinnamyl
carbonates.[12]
Reactions of 4a with heteroaryl and aliphatic allylic
carbonates, as well as a dienyl carbonate, also occurred with
moderate to high regioselectivity (5/6 = 77:23 to 99:1) and
excellent enantioselectivity (98–99% ee) (Table 1, entries 7–
10). These reactions occurred at room temperature and gave
the corresponding N-allylindoles in higher yields with higher
regioselectivities at room temperature than at 508C. The
reaction of the benzyloxy-substituted allylic carbonate 3j
(R = CH2OBn) occurred with somewhat lower branched-to-
linear selectivity, but nevertheless provides a route to
hydroxyalkyl-substituted indoles (Table 1, entry 10). The
reaction of 4a with the aliphatic carbonate 3k containing
branching alpha to the allyl unit (R = cyclohexyl) required
9
10
11
12
13
14
15
CHO Me
CHO
Ph
Me
-(CH)4-
[a] See the Supporting Information for experimental details. [b] Deter-
mined by 1H NMR analysis of the crude reaction mixture. [c] Yield of
isolated 5. [d] Determined by chiral HPLC methods. [e] Methyl cinnamyl
carbonate was used as the electrophile.
tron-donating and electron-withdrawing organic substituents,
as well as halogens at the 5-position, were suitable nucleo-
philes for the allylation reaction with tert-butyl cinnamyl
carbonate (Table 2, entries 1–4). The reactions of 5-subsituted
ethyl indole-2-carboxylates (4b–e) occurred with high
branched-to-linear selectivity (94:6 to 97:3) and enantiose-
lectivity (99% ee), and products 5l–o were isolated in high
yields (84–91%). In addition, indole-2-carboxaldehyde (4 f)
reacted with 3a to form indole 5p in 89% yield with 99% ee
(Table 2, entry 5).
3-Substituted indoles also reacted to give N-allylindoles in
high yields with excellent selectivities. The reactions of
indoles 4g–4j containing a variety of electron-withdrawing
substituents at the 3-position (Table 2, entries 6–9) occurred
with high branched-to-linear selectivity (94:6 to 98:2) and
excellent enantioselectivity (96–97% ee) to form N-allylin-
doles 5q–t in high yields (84–93%). The reaction of 3-
phenylindole (4k) with tert-butyl cinnamyl carbonate
(Table 2, entry 10) occurred with excellent selectivities, but
low conversion because of deactivation of the catalyst.
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Indoles containing substituents in both the 2- and 3-
positions also underwent selective iridium-catalyzed N-ally-
lation reactions (Table 2, entries 11–15). For example, reac-
tions of 3a with 2,3-substituted indoles containing an
electron-withdrawing substituent at either the 2- or 3-
position, such as 3-methylindole-2-carboxaldehyde (4l) or 2-
phenylindole-3-carboxaldehyde (4m), gave the correspond-
ing N-allylated products in high yields with excellent regio-
and enantioselectivities (Table 2, entries 11 and 12). The
presence of a strong electron-withdrawing
group at either the 2- or 3-position of a 2,3-
disubstituted indole was not even required
for N-allylation. The reactions of 2,3-diphe-
nylindole (4n), 3-methyl-2-phenylindole
(4o), and carbazole (4p) with 3a occurred
with 98:2 branched-to-linear ratio in each
case and gave N-allylindoles 5x–z in 88–95%
yield with 98–99% ee (Table 2, entries 13–
15).
Scheme 1. Regioselective and enantioselective N-allylation of 7-azain-
dole 7 with 3a in the presence of catalyst 2.
The parent indole and certain indoles
containing one phenyl or solely alkyl sub-
stituents did not undergo N-allylation, but 7-
azaindole did undergo productive N-allyla-
tion. Allylations of indole, 2-methylindole,
and 2-phenylindole with 3a catalyzed by 1 or
2 and Cs2CO3 at 508C occurred selectively at
the 3-position, whereas no allylation products
formed from the analogous reactions of 3-
methylindole and 2,3-dimethylindole. How-
ever, the reaction of 7-azaindole 7 with tert-
butyl cinnamyl carbonate (3a) occurred with
greater than 90:10 N1-to-C3 selectivity and
91:9 branched-to-linear selectivity, and the
branched N-allylazaindole 8 was isolated
from this reaction in 79% yield and 99% ee
(Scheme 1).
Scheme 2. Syntheses of enantioenriched 3-(1H-indol-1-yl)-N-methyl-3-arylpropan-1-amines
11 and 12a–c, dihydropyrrolo[1,2-a]indole 13, and indol-1-yl propanonic acid 14 from N-
allylindoles. N-Allylindoles: 5p, R1 =CHO, R2 =H, 99% ee; 5q, R1 =H, R2 =CO2Me,
97% ee; 5x, R1, R2 =Ph, 99% ee; 5y, R1 =Ph, R2 =Me, 99% ee; 5z, R1, R2 =-(CH)4-,
98% ee. Reaction conditions: a) 5q, 9-BBN, THF, ꢀ788C to RT, then H2O2, 3m NaOH,
EtOH, 08C to RT, 97%, 97% ee; b) KOH, MeOH, reflux; c) PhBr, reflux, 68% over two
steps; d) PPh3, CCl4, 08C!RT, 91%; e) MeNH2, EtOH, 908C, 95%, 97% ee; f) 5x–z,
[Cp2Zr(H)Cl], THF, RT, then CH3NHOSO3H, 608C (see Scheme 2 for results); g) 5p,
MeNHOH·HCl, NaOAc, THF, reflux, 83%, 9:1 regioselectivity, >99:1 d.r.; h) Zn, AcOH,
H2O, 608C, 87%, 99% ee; i) PhI(OAc)2, TEMPO, NaHCO3, MeCN/H2O (1:1), 81%,
97% ee. 9-BBN=9-borabicyclo[3.3.1]nonane, TEMPO=2,2,6,6-tetramethyl-1-piperidinyl-
oxy free radical.
Within the past five years, molecular scaffolds containing
3-(1H-indol-1-yl)-3-arylpropan-1-amine and 3-(1H-indol-1-
yl)-propanoic acid substructures have been identified as
promising lead compounds for medicinal chemistry. 3-(1H-
Indol-1-yl)-N-methyl-3-arylpropan-1-amines are potent dual-
acting norephinephrine and serotonin reuptake inhibitors,[9a]
while 3-(1H-indol-1-yl)-3-arylpropanoic acids and 3-(1H-
indol-1-yl)-3-alkylpropanoic acids are constituents of potent
inhibitors of the integrin avb3.[10] All of these structures
contain a stereocenter at the 3-position of the arylpropan-
amine, arylpropanoic acid, or alkylpropanoic acid subunit
that could be formed with control of absolute stereochemistry
by iridium-catalyzed allylic substitution. Furthermore, the
allyl moieties of the N-allylindole products provide a suitable
handle for straightforward elaboration into the requisite
propanamine or propanoic acid units.
The syntheses of 3-(1H-indol-1-yl)-N-methyl-3-arylpro-
pan-1-amines 11 and 12a–c are presented in Scheme 2. The
known monoamine reuptake inhibitor (R)-3-(1H-indol-1-yl)-
N-methyl-3-phenylpropan-1-amine (11) was prepared from
5q by a sequence including removal of the 3-alkoxycarbonyl
group by hydrolysis and decarboxylation, and standard
conversion of the alkene unit into an aminoethyl group by
hydroboration, oxidation, then conversion of the alcohol into
the chloride, and finally substitution with methylamine.
However, we also developed a more direct route using
hydrozirconation. Hydrozirconation of N-allylindoles 5x–z
and subsequent reaction of the alkylzirconium intermediate
with N-methyl hydroxylamine-O-sulfonic acid gave 3-(1H-
indol-1-yl)-N-methyl-3-arylpropan-1-amines 12a–c in a two-
step, one-pot sequence from the allylation product in 63–73%
yield.
We also showed that highly substituted and enantioen-
riched dihydropyrrolo[1,2-a]indoles, which occur in natural
products and other biologically active molecules,[15] are
readily synthesized from appropriately substituted N-allylin-
doles. For example, the reaction of (R)-1-(1-phenylallyl)-1H-
indole-2-carboxaldehyde (5p) with N-methylhydroxylamine
forms a transient nitrone that undergoes an intramolecular
dipolar cycloaddition with the allyl moiety of 5p to form a
tricyclic cycloadduct in 83% yield with excellent regio- and
diastereoselectivity.[16] Reductive cleavage of the N O bond
ꢀ
contained in the cycloadduct gave the highly substituted
dihydropyrrolo[1,2-a]indole 13, a ring-constrained analogue
of the 3-(1H-indol-1-yl)-N-methyl-3-arylpropan-1-amines, in
87% yield.
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120, 603; Angew. Chem. Int. Ed. 2008, 47, 593; e) D. A. Evans,
K. R. Fandrick, H.-J. Song, K. A. Scheidt, R. Xu, J. Am. Chem.
Soc. 2007, 129, 10029; f) D. A. Evans, K. R. Fandrick, H.-J. Song,
J. Am. Chem. Soc. 2005, 127, 8942.
Enantioenriched N-allylindoles are also precursors to
indolyl-1-yl propanoic acids, which are key subunits in a class
of integrin avb3 inhibitors.[10] Sequential hydroboration and
oxidation of N-allylindole 5q and then oxidation of the
alcohol with iodobenzene diacetate, TEMPO, and sodium
bicarbonate led to (R)-3-(3-(methoxycarbonyl)-1H-indol-1-
yl)-3-phenylpropanoic acid (14) in 79% yield over two steps.
In summary, we have developed a method for highly
regioselective and enantioselective N-allylation of indoles
with achiral, acyclic allylic electrophiles in the presence of a
metallacyclic iridium catalyst. These reactions encompass a
broad range of indole nucleophiles, as well as a variety of
unsymmetrical aryl, heteroaryl, and aliphatic allylic carbo-
nates and a dienyl carbonate. The enantioenriched N-
allylindole products are formed with consistently high enan-
tioselectivity (96–99% ee) and are readily transformed into 3-
(1H-indol-1-yl)-N-methyl-3-arylpropan-1-amines, dihydro-
pyrrolo-[1,2-a]indoles, and indol-1-yl propanoic acids. The
use of this reaction to prepare libraries of enantioenriched
indole derivatives to identify lead compounds for medicinal
chemistry will be the subject of future studies.
[5] For recent examples, see: a) W.-B. Liu, H. He, L.-X. Dai, S.-L.
You, Synthesis 2009, 2076; b) W.-B. Liu, H. He, L.-X. Dai, S.-L.
You, Org. Lett. 2008, 10, 1815; c) H. Y. Cheung, W.-Y. Yu, F. L.
Lam, T. T.-L. Au-Yeung, Z. Zhou, T. H. Chan, A. S. C. Chan,
Org. Lett. 2007, 9, 4295; d) B. M. Trost, J. Quancard, J. Am.
Chem. Soc. 2006, 128, 6314; e) M. Bandini, A. Melloni, F.
Piccinelli, R. Sinisi, S. Tommasi, A. Umani-Ronchi, J. Am.
Chem. Soc. 2006, 128, 1424.
[6] M. Bandini, A. Eichholzer, M. Tragni, A. Umani-Ronchi,
Angew. Chem. 2008, 120, 3282; Angew. Chem. Int. Ed. 2008,
47, 3238.
[7] H.-L. Cui, X. Feng, J. Peng, J. Lei, K. Jiang, Y.-C. Chen, Angew.
Chem. 2009, 121, 5847; Angew. Chem. Int. Ed. 2009, 48, 5737.
[8] B. M. Trost, M. J. Krische, V. Berl, E. M. Grenzer, Org. Lett.
2002, 4, 2005.
[9] a) P. E. Mahaney, A. T. Vu, C. C. McComas, P. Zhang, L. M.
Nogle, W. L. Watts, A. Sarkahian, L. Leventhal, N. R. Sullivan,
A. J. Uveges, E. J. Trybulski, Bioorg. Med. Chem. 2006, 14, 8455;
b) “3-Amino-1-arylpropyl Indoles as Monoamine Reuptake
Inhibitors”: R. Greenhouse, S. Jaime-Figueroa, L. Raptova,
D. C. Reuter, K. A. Stein, R. J. Weikert, Int. Pat. Appl. WO
2005/118539A1, 2005.
Received: August 3, 2009
[10] K. Leonard, W. Pan, B. Anaclerio, J. M. Gushe, Z. Guo, R. L.
DesJarlais, M. A. Chaikin, J. Lattanze, C. Crysler, C. L. Manthey,
B. E. Tomczuk, J. J. Marugan, Bioorg. Med. Chem. Lett. 2005, 15,
2679.
Published online: September 16, 2009
Keywords: allylation · asymmetric catalysis · heterocycles ·
.
iridium · synthetic methods
´
[11] D. Markovic, J. F. Hartwig, J. Am. Chem. Soc. 2007, 129, 11680.
[12] L. M. Stanley, J. F. Hartwig, J. Am. Chem. Soc. 2009, 131, 8971.
[13] C. A. Kiener, C. Shu, C. Incarvito, J. F. Hartwig, J. Am. Chem.
Soc. 2003, 125, 14272.
[1] For reviews, see: a) L. Gupta, A. Talwar, P. M. S. Chauhan, Curr.
Med. Chem. 2007, 14, 1789; b) R. Cao, W. Peng, Z. Wang, A. Xu,
Curr. Med. Chem. 2007, 14, 479; c) S. E. Lewis, Tetrahedron 2006,
62, 8655; d) S. E. OꢀConnor, J. J. Maresh, Nat. Prod. Rep. 2006,
23, 532; e) W. Gul, M. T. Hamann, Life Sci. 2005, 78, 442; f) A.
Aygꢁn, U. Pindur, Curr. Med. Chem. 2003, 10, 1113.
´
[14] S. T. Madrahimov, D. Markovic, J. F. Hartwig, J. Am. Chem. Soc.
2009, 131, 7228.
[15] a) R. M. Wilson, R. K. Thalji, R. G. Bergman, J. A. Ellman, Org.
Lett. 2006, 8, 1745; b) R. Peters, P. Waldmeier, A. Joncour, Org.
Process Res. Dev. 2005, 9, 508; c) “Indole Derivatives and their
use as 5-HT2B and 5HT2C Receptor Ligands”: J. M. Bentley, M. J.
Bickerdike, P. Hebeisen, G. A. Kennett, S. Lightowler, P. Mattei,
J. Mizrahi, T. J. Morley, J.-M. Plancher, H. Richter, S. Roever, S.
Taylor, S. P. Vickers, Int. Pat. Appl. WO 02/051844A1, July 4,
2002; d) “Pyrroloindoles, Pyridoindoles, and Azopinoindoles as
5-HT2C Agonists”: D. R. Adams, J. M. Bentley, J. R. A. Roffey,
R. J. Hamlyn, S. Gaur, M. A. J. Duncton, J. E. P. Davidson, M. J.
Bickerdike, I. A. Cliffe, H. L. Mansell, US Patent 6,433,175 B1,
2002; e) M. B. Johansen, M. A. Kerr, Org. Lett. 2008, 10, 3497;
f) H. Kusama, J. Takaya, N. Iwasawa, J. Am. Chem. Soc. 2002,
124, 11592.
[16] a) E. M. Beccalli, G. Broggini, A. Farina, L. Malpezzi, A.
Terraneo, G. Zecchi, Eur. J. Org. Chem. 2002, 2080; b) E. M.
Beccalli, G. Broggini, C. La Rosa, D. Passarella, T. Pilati, A.
Terraneo, G. Zecchi, J. Org. Chem. 2000, 65, 8924; c) B. Baruah,
D. Prajapati, A. Boruah, J. S. Sandhu, Synth. Commun. 1997, 27,
2563; d) D. St. C. Black, D. C. Craig, R. B. Deb-Das, N. Kumar,
Aust. J. Chem. 1993, 46, 603; e) P. J. Bhuyan, R. C. Boruah, J. S.
Sandhu, Tetrahedron Lett. 1989, 30, 1421.
[2] For reviews, see: a) T. B. Poulsen, K. A. Jørgensen, Chem. Rev.
2008, 108, 2903; b) M. Bandini, A. Eichholzer, A. Umani-
Ronchi, Mini-Rev. Org. Chem. 2007, 4, 115; c) M. Bandini, S.
Tommasi, A. Umani-Ronchi, Synlett 2005, 1199; d) M. Bandini,
A. Melloni, A. Umani-Ronchi, Angew. Chem. 2004, 116, 560;
Angew. Chem. Int. Ed. 2004, 43, 550; S.-L. You, Q. Cai, M. Zeng,
Chem. Soc. Rev. 2009, 38, 2190.
[3] For recent examples, see: a) Y.-X. Jia, J. Zhong, S.-F. Zhu, C.-M.
Zhang, Q.-L. Zhou, Angew. Chem. 2007, 119, 5661; Angew.
Chem. Int. Ed. 2007, 46, 5565; b) G. B. Rowland, E. B. Rowland,
Y. Liang, J. A. Perman, J. C. Antilla, Org. Lett. 2007, 9, 2609;
c) Q. Kang, Z.-A. Zhao, S.-L. You, J. Am. Chem. Soc. 2007, 129,
1484; d) M. Terada, K. Sorimachi, J. Am. Chem. Soc. 2007, 129,
292; e) H. Li, Y.-Q. Wang, L. Deng, Org. Lett. 2006, 8, 4063;
f) Y.-Q. Wang, J. Song, R. Hong, H. Li, L. Deng, J. Am. Chem.
Soc. 2006, 128, 8156.
[4] For recent examples, see: a) M. Ganesh, D. Seidel, J. Am. Chem.
Soc. 2008, 130, 16464; b) P. K. Singh, V. K. Singh, Org. Lett. 2008,
10, 4121; c) J. Itoh, K. Fuchibe, T. Akiyama, Angew. Chem. 2008,
120, 4080; Angew. Chem. Int. Ed. 2008, 47, 4016; d) M. Rueping,
B. J. Nachtsheim, S. A. Moreth, M. Bolte, Angew. Chem. 2008,
7844 www.angewandte.org
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